There was one study where a drug was designed to improve motor response in patients with retarded motor skills. The doctors had found a way to determine which patient was receiving which drug.
So when it came time for the doctors to measure efficacy of the drug, they would do the motor skills test. If the patient was on the actual drug, and didn't show improvement, the doctors had a tendency to say things like "Well, that can't be right. Let me run the test again."
Disaster. Hundred million dollar trial down the toilet.